Propellants with improved burning rate



3,918,264 Patented Jan. 23, 1962 3,018,204 PROPELLANTS WITH IMPROVEDBURNING RATE Charles C. Bice, Bartlesville, Okla, assignor to PhillipsPetroleum Company, a corporation of Delaware No Drawing. Filed June 23,1958, Ser. No. 743,957 8 Claims. (Cl. 149-19) This invention relates toburning rate.

In the preparation of propellants, particularly propellants for use inmissiles, maximum performance per unit weight of propellant is desired.One drawback in the acceptance of solid propellants has been a lowerburning rate than that obtainable with liquid propellants. The presentinvention relates to a propellant composition which has an improvedburning rate and wherein the performance per unit weight is materiallyincreased as a result of a synergistic effect of ingredients in thepropellant.

The following are objects of my invention.

An object of this invention is to provide a propellant having a fastburning rate. A further object of my invention is to improve theperformance of propellants. A further object of my invention is toprovide a propellant comprising a binder, an oxidant, and which containsboron carbide and activated charcoal.

Other objects and advantages of my invention will be apparent to oneskilled in the art upon reading this disclosure.

Broadly, the invention is directed to solid propellants using a bindersuch as asphalt, various resins, plastics, orrubber. A solid inorganicoxidizing salt is used as the oxidant. The improvement is based upon thediscovery that the performance of the propellant can be improved byincorporating, on a weight basis, 0.1 to 30, preferably 0.5 to 10, partsof finely divided boron carbide per 100 parts of binder plus oxidizerand 0.1 to 10 parts of activated charcoal, preferably 0.2 to parts, perpart of said boron carbide. The total weight of carbide and charcoalshould be less than 110 parts per 100 parts of binder plus oxidizer. Theaddition of the boron carbide improves the burning rate results in anincrease in performance per unit weight when compared to the samepropellant without the carbide.

Synthetic rubbers and particularly the polymers prepropellants withimproved pared by polymerizing a major amount of 1,3-butadiene withcopolymerizable monomers, such as styrene and various vinylpyridines,are frequently preferred polymers for use in preparation of the binder.The polymers of 1,3 butadiene and 2-methyl-S-vinylpyridine, such as the90/ copolymer have been widely used.

As those skilled in the art will understand, the conjugated dienes whichcan be employed are, in addition to butadiene, those which contain from4 to 6, inclusive, carbon atoms per molecule and include 1,3-butadiene,isoprene (2-methyl-1,3-butadiene), piperylene, 2-methyl-1,3- pentadiene,2,3-dimethyl-l,3-butadiene, and others. Mixtures of conjugated dienescan be used.

The polymerizable heterocyclic nitrogen bases which are applicable inour invention are those of the pyridine and quinoline series which arecopolymerizable with a conjugated diene and contain one and only onesubstituent wherein R is either hydrogen or a methyl group. That is, thesubsti-tuent is either a vinyl or an alpha-methylvinyl (isopropenyl)group. Of these compounds, the pyridine derivatives are of the greatestinterest commercially at present. Various substituted derivatives arealso applicable but the total number of carbon atoms in the nuclearsubstituted groups, such as alkyl groups, in addition to the vinyl oralpha-methylvinyl group, should not be greater than 12 because thepolymerization rate decreases somewhat with increasing size of the alkylgroup. Compounds where the alkyl substituents are methyl and/or ethylare available commercially.

These heterocyclic nitrogen bases have the formula where R is selectedfrom the group consistingof hydrogen, alkyl, vinyl, alpha-methylvinyl,alkoxy, halo, hydroxy, cyano, aryloxy, aryl, and combinations of thesegroups such as haloalkyl, alkylaryl, hydroxyaryl, and the like; one andonly one of said groups being selected from the group consisting ofvinyl and alpha-methylvinyl; and the total number of carbon atoms in thenuclear substituted groups in addition to the vinyl or alpha-methylvinylgroup, being not greater than 12. Examples of such compounds are2vinylpyridine; 2-vinyl-5rethylpyridine; Z-methyl-S-vinylpyridine;4-vinylpyridine; 2,3,4-trimethyl- 5-vinylpyridine; 3,4,5,6tetramethyl-Z-vinylpynidine; 3- ethyl-S-VinyIpyridine; 2,6diethyl-4-vinylpyridine; 2-isopropyl-4-nonyl-5-vinylpyridine; Z-methyl 5undecyl-3- vinylpyridine; 2,4 dimethyl-5,6-dipentyl-3-vinylpyridine;Z-decyl-S- (alpha-methylvinyl pyridine; 2-vinyl-3-methyl 5ethylpyridine; 2 methoxy 4 chloro 6 vinyl pyridine;3-vinyl-5-ethoxypyridine; 2-vinyl-4,5-dichloropyridine; 2-alpha-methylvinyl -4-hydroxyl6-cyanopyridine; 2-vinyl-4-phenoxy 5methylpyridine; 2-cyano-5- (alpha-methylvinyl)pyridine;3-vinyl-5-phenylpyridine; 2-(para-methylphenyl)-3-vinyl-4-methylpyridine; 3-vinyl-5-(hydroxyphenyl)-pyridine; 2-vinylquinoline; 2-vinyl-4- ethylquinoline;3-vinyl-6,7-di-n-propylquinoline; 2-methyl- 4 nonyl 6 vinylquinoline;4-(alpha-methylvinyl)-8-' dodecylquinoline; 3-vinylisoquinoline;1,6-dimethyl-3- vinylisoquinoline; 2 vinyl 4-benzylquinoline; 3-vinyl-5-chloroethylquinoline; 3-vinyL5,6-dichloroisoquinoline; 2-vinyl-6-ethoxy-7-methylquin0line; 3-vinyl 6 hydroxymethylisoquinoline;and the like. Mixtures can be used. The copolymers are prepared bypolymerizing a major amount of the diene with a minor amount of theheterocyclic nitrogen base by any suitable method. Preferably, theultimate product has a Mooney viscosity (ML-4) of 10 to 40 althoughhigher Mooney polymers are frequently made and then mixed with extenderoils. t

In the preparation of propellants in accordance with this invention asuitable binder can be prepared using these rubbery polymers inconjunction with well-known rubber compounding ingredients. Carbon blackis commonly used as a rubber reinforcing agent and as a fuel in amountsto 100 parts and more for each 100 parts of rubber. Stabilizers,softeners, sulfur or other curative systems are mixed with the polymerto give binder compositions which can be vulcanized by heating.Vulcanization is effected after admixing the binder with the oxidizerand other propellant ingredients by heating, commonly to temperatures inthe range of 150 to 300 F. for l to 48 hours. Sulfur cure is notrequired and other methods can be used. Cure by quaternization is onesuch sulfur-free cure. This method is disclosed in Reynolds andP-nitchard application Serial No. 284,447, filed April 25, 1952.

In one method of preparing the propellant, a solvent mix technique isused. In this process the binder is dispersed in a solvent such as,n-hexane, cyclohexane, methylcyclohexane, and the like. The oxidizersand other propellant components are mixed with the solvent-bindermixture. When a uniform blend has been obtained the solvent is removedand the propellant formed by molding. Conventional milling procedurescan also be used.

By boron carbide I mean compositions represented by the formula BxCywhere the ratio x/ y varies between 1.0 and 6.0. The preparation andproperties of these boron carbides have been reviewed by Glaser andMoskowitz, Ioumal of Applied Physics, 24, No. 6, pages 731-3 (June1953). These compounds are described as solid solutions of varyingamounts of carbon in a slightly distorted boron lattice. The values forthe ratio of x/y need not therefore be a whole number within the rangeof 1.0 to 6.0. Examples of compounds which are useful in the practice ofthis invention can be represented as BC, B C, B C, B C, B C. Mixtures ofthese compounds can also be utilized. Carbides having a particle sizeless than about 200 microns, preferably less than 100 microns, arepreferred.

The'preferred activated charcoal for use in the practice of thisinvention is material designated as decolorizing or deodorizing charcoalor carbon. Suitable activated charcoals are prepared from variouscarbonaceous products including wood, sawdust, peat, fruit pits,lignite, pulpmill waste, coconut shells, blood, bone and coal. Theseproducts are carbonized in the absence of air at temperatures belowabout 600 C. Carbonization is also effected in the presence of steam orcarbon dioxide at temperatures of 800 to 900 C. The carbonized productis preferably ground to a size less than about 400 microns.

The oxidizers which are used in the preparation of the propellantinclude ammonium nitrate, potassium nitrate, ammonium perchlorate,potassium perchlorate and other salts of nitric and perchloric acid,These oxidizers can be used alone or. in admixture. Mixtures of theseo-xidizerswith other modifiers can be desirable. Thus, I can use inadmixture with the oxidizer; metallic oxides, such as PbO metallicnitrates, such as Ba(NO metallic chromates and bichromates, such as(NI-10 G KgCr o and K CrO metal powders, such as aluminum, magnesium,iron and boron.

One particularly suitable oxidizer is phase stabilized ammonium nitrate.Phase stabilized ammonium nitrate can be prepared by admixing theammonium nitrate with potassium nitrate, and 1) cocrystallizing thenitrates from water solution; (2) heating a moist mixture at 50 C.; and(3) by fusing a physical mixture of the nitrates. Commonly, the phasestabilized ammonium nitrate is prepared with about 5 to about 15 percentby weight potassium nitrate.

For the'practiceof my invention I prefer to mix the boron carbide andthe activated charcoal carbon with the oxidizer. This mixture is thenblended with the binder to achieve 'a uniform mixture. Mixing can beeffected conveniently in an internal mixer, such as the Baker-Perkinsand the Banbury mixer and on a roll mill.

In the process of making the propellant the binder is commonly used inamounts from about 3 to about 25 parts, to correspondingly 97 to 75parts of oxidizer or mixtures of oxidizers, exclusive of othermodifiers. For each 100 parts by weight of binder pilus oxidizer, Iprefer to use 0.5 to 10 parts by Weight of boron carbide althoughsmaller and greater amounts can give beneficial effects. For example,noticeable improvement is observed with as little as 0.1 part of thecarbide. Amounts greater than 10 parts, for example 20 to 30 parts, aresometimes less effective per unit weight. For each part of boroncarbide, I prefer to use at least 0.1 part to 10 parts, more preferably0.2 to 5 parts, of activated charcoal. The total weight of carbide pluscharcoal is kept below parts per 100 parts of binder plus oxidizer. Theactivated charcoal with the boron carbide permits formulation ofpropellants having enhanced burning rates. The significance of thisinvention is illustrated by the following example.

Example Propellant compositions were mixed and burning rate strands wereprepared according to the procedure given below. For the binder, arubbery 90/10 copolymer of 1,3-butadiene/2-methyl-5-vinylpyridine wasprepared, blended with carbon black and stabilizer. This binder wasmixed with the oxidizer (phase stabilized ammonium nitrate), cathalyst(ammonium dichromate), and other additives (boron carbide and activatedcharcoal) in the process of preparing the propellant strands. Details ofthe process are given below. p v

The rubbery polymer was prepared by emulsion poly merization at 41 F. of90 parts 1,3-butadiene and 10 parts of 2 methyl-5- vinylpyridine. Therubbery polymer was latex masterbatched with furnace carbon black andtreated with antioxidant.

composition: I

Parts by weight. 90/10 copolymer 100 Carbon black (Philblack A) 22Antioxidant (Flexamine) 3 The oxidizer was prepared by blending 90 partsby weight of ammonium nitratewith 10 parts by weight of potassiumnitrate. A small amount (about 0.2 part per 100 parts of the oxidizers)of water was added and the mixture was blended. The product was thendried in'air at a temperature of about l60 F. and ground to a particlesize less than about 150 microns. The weight average particle size wasabout 50 to 60 microns. oxidizer was mixed with other dry ingredients.

The boron carbide, B C, was a commercial product substantially less thanabout 44 microns (325 mesh) in size. having a weight average particlesize of about 18 microns. The activated charcoal, Norit- A, was a' dry,powdery product.

The propellant was formed by the solvent mix technique. By this methodthe rubbery binder was first slurried in diameter, were cut from theslabs. The burning" rate was measured at a temperature of 70 F. by theCrawford bomb test. In this test the strands are burned in a nitrogenatmosphere and the rate of burning in inches per second is measured. Thestrand burning rate at a given pressure is obtained from agraph of theresults obtained at several pressures.

The resulting binder had the The The ammonium dichromate wa agroundproduct The compositions which were prepared and their strandburning rates are set forth below.

Composition 1 shows that in the absence of either the activated charcoalor boron carbide, the strand burning rate is 0.217 in./sec. As shown bycompositions 2 and 3, the charcoal or carbide alone increases theburning rate by 0.040 in./sec. to a value of 0.257 in./sec. With thecombination of activated charcoal and boron carbide, as with composition4, the burning rate is 0.362 in./sec. If one plots the burning rateversus parts by weight of activated charcoal in the propellant for runs1 and 2, the burning rate with 3 parts of charcoal can be obtained byextrapolation. This value is 0.273 in./sec., as compared with the valueof 0.362 actually found for composition 4. Thus, the high rate for thisrun is attributed to a synergistic effect of the activated charcoal andboron carbide.

In addition to the Norit A of the example, other activated charcoals aresuitable, Commercially available at the present time are Nuchar andDarco.

As many possible embodiments can be made of this invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth is to be interpreted as illustrative and not as undulylimiting the invention.

I claim:

1. A propellant composition containing the following in parts by weightBinder 3-25 Solid inorganic oxidizing salt 97-75 Binder 3-25 Solidinorganic oxidizing salt 97-75 wherein said binder is selected from thegroup consisting of asphalt and rubber, the amount of binder andoxidizing salt being sufficient to total parts by weight, 0.5 to 10parts by Weight of boron carbide per 100 parts by Weight of binder plusoxidizing salt, and 0.2 to 5 parts by weight of activated charcoal perpart by Weight of boron carbide, the total Weight of boron carbide pluscharcoal not exceeding parts by Weight per 100 parts by Weight of binderplus oxidizing salt.

3. The propellant of claim 1 wherein said binder is a rubbery copolymerof a conjugated diene containing 4 to 6 carbon atoms and styrene.

4. The propellant of claim 1 wherein said binder is a rubbery polymer ofbutadiene and styrene.

5. The propellant of claim 1 wherein said binder is a rubbery copolymerof a conjugated diene of 4 to 6 carbon atoms and a compound selectedfrom the group consisting of pyridine and quinoline containing a whereinsaid binder has the following composition Parts by weightButadiene/2-methyl-5-vinylpyridine copolymer 100 Furnace black 22Antioxidant 3 References Cited in the file of this patent UNITED STATESPATENTS Moore et al May 14, 1957 Fox Mar. 17, 1959 OTHER REFERENCESChem. and Eng. News, Oct. 7, 1957, pages 62-63,

1. A PROPELLANT COMPOSITION CONTAINING THE FOLLOWING IN PARTS BY WEIGHTBINDER 3-25 SOLID INORGANIC OXIDIZING SALT 97-75 WHEREIN SAID BINDER ISSELECTED FROM THE GROUP CONSISTING OF ASPHALT AND RUBBER, THE AMOUNT OFBINDER AND OXIDIZING SALT BEING SUFFICIENT TO TOTAL 100 PARTS BY WEIGHT,0.1 TO 30 PARTS BY WEIGHT OF BORON CARBIDE PER 100 PARTS BY WEIGHT OFBINDER PLUS OXIDIZING SALT, AND 0.1 TO 10 PARTS BY WEIGHT OF ACTIVATEDCHARCOAL PER PART BY WEIGHT OF BORON CARBIDE, THE TOTAL WEIGHT OF BORONCARBIDE PLUS CHARCOAL NOT EXCEEDING 110 PARTS BY WEIGHT PER 100 PARTS BYWEIGHT OF BINDER PLUS OXIDIZING SALT.